In many applications, it is desirable to minimize the current drawn by a multiphase induction motor during starting. Starting, or in-rush, current for multi-phase motors tends to be 5-6 times the running current. Such high currents have many detrimental effects on the equipment and the power, as well as the economics of power usage. By means of example only, drawing such normal starting currents over a long power line can cause the voltage to essentially collapse, leaving insufficient voltage to accomplish the task. Furthermore, other customers along the same power line may experience undesirable voltage fluctuations during the start of the motor. To discourage this situation, power companies, especially in Europe, impose penalties if a customer's starting or in-rush currents are excessive.
Use of an autotransformer is one known methodology for achieving lower motor starting currents. Autotransformers, however, are relatively inflexible compared to the present invention in that the turns ratio of an autotransformer is established up front and remains fixed by the design of the components. Another approach employs the use of series elements such as inductors, resistors, and the like, to limit starting current. The latter approach, however, requires significantly higher line currents than autotransformer starters to provide the same amount of torque.
Recent developments in the power electronics technology provides for broad utilization of power semiconductor devices, such as IGBTs (i.e., insulated-gate bipolar transistors) or MCTs (i.e., MOS-controlled thyristors). These solid state devices can be used to provide an improved means for reduced voltage start-up control of commonly used three-phase induction motors as mentioned in the prior art. For example, U.S. patent application Ser. No. 08/852,502, titled "A Solid State, Reduced Voltage Motor Starter Enabling Equalized Input/Output VA Rating" and filed on May 7, 1997, in the names of Frank Wills and Harold Schnetzka, the specification of which is hereby incorporated by reference in its entirety, discloses and claims a solid state starter circuit for a multi-phase motor that minimizes and selectively controls the amount of line current drawn by the motor during starting. That starter circuit, however, while an improvement over other known reducedcurrent starter circuits and methodologies, including autotransformers, requires the use of an energy current sink which stores or dissipates energy trapped in the motor windings during certain periods of each line cycle while the motor is starting up.
It is desirable, therefore, to provide an improved solid state motor starter with energy recovery, such that the use of a storage or dissipative device is not required. More particularly, it is desirable to recover the energy trapped in the motor windings so as to reduce the net energy drawn from the power lines to start the motor.